1. Field of the Invention
The invention relates in general to a transformer, and more particularly to a transformer applied in a lamp driving system.
2. Description of the Related Art
Along with the coming multi-media era, liquid crystal displays (LCD) have increasingly been applied to the products, such as computer monitors and LCD TVs. Generally, the LCD utilizing a light and high-performance lamp driving system to generate backlight has advantages of having a thin body and displaying clear/stable pictures. The lamp driving system in a LCD is mainly composed of a discharge lamp, such as a cold cathode fluorescent lamp (CCFL), and a transformer for driving the CCFL.
Referring to FIG. 1A and FIG. 1B at the same time, an exploded view of a conventional transformer and a combined view of the conventional transformer are respectively shown. The conventional transformer 100 is used to drive the CCFL in a LCD. The transformer 100 includes a bobbin 102, a first E-shape magnetic core 104 and a second E-shape magnetic core 106.
The primary side coil and the secondary side coil (not shown in the figure) are wound around the bobbin 102, and two ends of the bobbin 102 have a number of metal pins 108 for connecting to the primary side coil and the secondary coil and welding to a circuit board.
The first E-shape magnetic core 104 and the second E-shape magnetic core 106 are combined together along the direction shown by the arrow in FIG. 1A to become a combined transformer 100 as shown in FIG. 1B. The combined first and second E-shape magnetic cores 104 and 106 respectively generate a magnetic flux in the exterior and the interior of the bobbin 102.
Referring to FIG. 2, a vertical view of the conventional transformer 100 is shown. The bobbin 102 of the transformer 100 is not shown in FIG. 2 to clarify the figure. From the figure, it can be seen that the first E-shape magnetic core 104 and the second E-shape magnetic core 106 are combined together. The primary side coil 210 and the secondary side coil 212 are wound around the bobbin. That is, the primary side coil 210 and the secondary side coil 212 are respectively wound around the central parts of the first E-shape magnetic core 104 and the second E-shape magnetic core 106.
When an alternating voltage signal is input to the primary side coil 210, the magnetic flux is generated in the combined first and second E-shape magnetic cores 104 and 106 as shown by the dotted lines in the figure. Due to induction of the magnetic flux, another alternating voltage signal is output from the secondary side coil 212, which is utilized to drive the CCFL.
The lamp driving system of a LCD is generally covered by a metal housing to prevent the EMI it generates from interfering with the LCD panel. However, due to the tendency of LCD to be thin, the distance between the metal housing and the transformer 100 is quite small.
In the conventional transformer 100, as shown by the dotted line in FIG. 2, the magnetic flux generated by inputting an alternating voltage signal to the primary side coil 210 and originally exiting in the first E-shape magnetic coil 104 and the second E-shape magnetic coil 106, is partially spread to the metal housing, thereby causing the flux leakage and magnetic flux loss of the transformer 100 due to inadequate distance between the metal housing and the transformer 100. The magnetic flux loss of the transformer 100 will increase the loading effect and cause the bobbin current not easily balanced, the voltage signal output by the secondary side coil 212 unstable, and the CCFL driven by the transformer 100 to illuminate unstably, thereby influencing the display quality of the LCD.